A lot of people don't, and often they have not even read the whole holy book they claim to follow.
Both believers and unbelievers typically do not grasp spiritual concepts well.
They ether have to make it all literal or all symbolic. They become too legalistic or too permissive. They are seeking to reconcile their society's ideals with the spiritual concepts, and it cannot be done, IMO.
For example, many will see Biblical God (especially in the Hebrew OT) as demanding and/or allowing/causing/condoning atrocities instead of yielding to humans, aka, giving us the freedom to be wrong and discover why (the book of Judges is a good example this).
When I shifted away from the knee-jerk reactions many get stuck on, then I deepened my understanding in a way that
Most of the points people try to make about the Bible fail to grasp it, IMO, such as arguments that it gives slavery a positive value or the misunderstanding that the Mosaic Law was not intended to be universal or timeless or perfect (how can imperfect people be governed by a perfect law? instead, their imperfection is being mitigated perfectly), but existed within a very specific culture and time period. Basically, others are arguing against a strawman, which is why they have no effect on me. When people try to tell me how I feel or what I think, the I admittedly shut off to them. People trying to tell me how I understand something is ridiculous. Usually they are arguing stuff I don't believe and is not my understanding of scripture, and most of the time I am better versed in it than they are, because they tend to "cherry pick".
I certainly have had my own doubt, currently have doubts crop up, etc, and I usually resolve them by shifting my viewpoint, often shifting it away from the usual interpretations. Doing this requires I detach myself from current social mores also, and I try to understand the Bible in its own context. This has left me with views very different from most of mainstream Christianity in some ways. However, I do not form an acceptance of trendy values at all, because I don't seek to reconcile "progressive" ideas of morality and ethics with the Bible. Instead, I take both a less legalistic and a less permissive view and see it more about spiritual principles (understanding the dynamics of the spiritual aspect of reality and why certain actions cause certain outcomes, which is why they can be assigned moral values; my metaphor for this is the law of gravity). I am less focused on understanding "what" than "why".
I also find there is a lot of human arrogance regarding "facts", asserting things as true that 20 years from now will change. When it comes to pre-history and holy books, such as the Bible, I see no reason to try and resolve its accounts with "facts", as the facts are not static; but I find the spiritual principles to be timeless. The timelessness is hard for others to grasp, even if they recognize it to a degree, which is why they are always trying to reconcile it to their particular society. They see value in it, but they don't grasp it well enough to not take it too literally, which presents obvious conflicts.
So I don't regard the Bible as a scientific nor historical book, although that doesn't mean I think it is a book of fables. I simply don't believe history and science to be static either, but rather very contextual and shifting as our collective perspective shifts (and this is certainly true of religion also, which I don't equate with the Bible or spirituality itself; see above about people trying to "reconcile"). It is something of a paradox that to understand the spiritual principles in the Bible outside of its context, you have to first grasp it within its context. It is something of a paradox in that to believe, you have to admit that humans (including yourself) don't really know much at all.
For example... Did Noah's flood really happen? Well, I don't know. I wasn't there. The way people understood and presented things back then is pretty different from how reality is interpreted now. The current & past facts become irrelevant, but the spiritual lesson stands. There is no need to reconcile present understanding of the physical world and history with the past understanding of the physical world and history. So when it comes to belief, I believe fully in the Bible as a book of spiritual truths. I have no need to believe it as "factual" in a physical sense, as that would miss the point of it anyway.
I am not looking to debate here, but this is simply my perspective.